Make sure the UPVC door frame is absolutely level and plumb and use a straight edge for checking bow on the frame.Also check for corner to corner for square as it is very important when fitting the door.

And finally make sure your spirit level is level !!!! ( This will guarantee to stop the swearing box filling up later on ! )

You can test this by putting the spirit level on any level + upright surface and turn the spirit level round again on the same spot to check the bubble is in the same place,if not adjust the level.